Premix burners for the combustor of a gas turbine are disclosed e.g. in the documents EP 0 321 809 and EP 0 704 657.
In particular, said premix burners can be operated with liquid and/or gaseous fuels of all kinds. Thus, it is possible to operate burners or combustors simultaneously or in sequence with different fuels.
Gas turbines with sequential are shown in EP 0 620 362 A1 or DE 103 12 971 A1.
Additional relevant state of the art is disclosed in the following documents.
EP 0 321 809 A1 relates to a burner consisting of two hollow conical half-shells, forming tangential air inlet slots and feed channels for gaseous and liquid fuels, whereby the center axes of the hollow conical half-shells run in the longitudinal direction at a mutual offset. A fuel nozzle, which is located in the middle of the connecting line of the mutually offset center axes of the half-shells, is placed at the burner head within the conical interior space formed by the half-shells.
EP 0 704 657 A1 describes a burner arrangement for a heat generator, substantially consisting of a swirl generator, as disclosed in EP 0 321 809 A1, for a combustion air flow and means for injecting fuel into said air flow, and a mixing zone downstream of said swirl generator for homogenously mixing fuel and combustion air before introducing the fuel-air-mixture into a combustion chamber, where ignition occurs.
It is a fact, that referring to gas turbine engine a flexibility with respect to the operational process is required. This implies that gas turbine engines often operate at lower load than the base load design point, i.e. at lower combustor inlet and firing temperatures.
At the same time, emission limit values and overall emission permits are becoming more stringent, so that it is required to operate at lower emission values, keep low emissions also at part load operation and during transients.
State-of-the-art combustion systems are designed to cope with a certain variability in operating conditions, e.g. by adjusting the compressor inlet mass flow or controlling the fuel split among different burners, fuel stages or combustors. However, this is not sufficient to meet the new requirements.
To further reduce emissions referring to the operational flexibility, a sequential combustion has been suggested in DE 103 12 971 A1. Depending on the operating conditions, in particular on the hot gas temperature of a first combustion chamber, it can be necessary to cool the hot gases before they are admitted to a second burner (also called sequential burner). This cooling can be advantageous to allow fuel injection and premixing of the injected fuel with the hot flue gases of the first combustor in the second burner.
Conventional cooling methods either require heat exchangers which lead to high pressure drops in the main hot gas flow or suggest injection of a cooling medium from the side walls. For injection of a cooling medium from the side walls a high pressure drop is required which is detrimental to the efficiency of a gas turbine operated with such a combustor arrangement and a controlled cooling of the whole flow is difficult.
With reference to WO 2014/063835 A1 a sequential combustor arrangement is disclosed, comprising a first burner, a first combustion chamber, a mixer for admixing a dilution gas to the hot gases leaving the first combustion chamber during operation, a second burner, and a second combustion chamber arranged sequentially in a fluid flow direction. The mixer is adapted to guide combustion gases in a hot gas flow path extending between the first combustion chamber and the second burner comprising a duct having an inlet at an upstream end adapted for connection to the first combustion chamber and an outlet at a downstream end adapted for connection to the second burner.
The mixer comprises a plurality of injection pipes pointing inwards from the side walls of the mixer for admixing the dilution gas to cool the hot flue gases leaving the first combustion chamber.
In addition, WO 2014/063835 A1 describes a method for operating a gas turbine with at least a compressor, a combustor arrangement comprising a first burner, a first combustion chamber, a mixer for admixing a dilution gas to the hot gases leaving the first combustion chamber during operation, a second burner and a second combustion chamber arranged sequentially in a fluid flow connection. The mixer is adapted to guide combustion gases in a hot gas flow path extending between the first combustion chamber and the second burner comprising a duct having an inlet at an upstream end adapted for connection to the first combustion chamber and an outlet at a downstream end adapted for connection to the second burner. The mixer comprises a plurality of injection pipes pointing inwards from the side walls of the duct for admixing the dilution gas to cool the hot flue gases leaving the first combustion chamber, and a turbine. The dilution gas is admixed into different regions of the cross section of the mixer or the dilution gas is injected through injection holes and/or second injection pipes and first injection pipes in order to introduce dilution gas into different regions of the cross section of the mixer.
Moreover, the solution according to WO 2014/063835 A1, referring to the cross-flow injection system, offers also the advantage of a simpler geometry and lower cost. However, such solution needs longer mixing length and is much more sensitive with respect to changes in the hot gas temperature of the first combustor and dilution air temperature, associated with the change in engine load.